1. Field of the Invention
The present invention relates to wireless communication systems, wireless communication devices, and wireless communication methods which are capable of performing spatial multiplexing transmissions on a plurality of streams on the basis of channel matrixes using channel transfer functions of pairs of a transmission antenna and a reception antenna. The present invention particularly relates to a wireless communication system, a wireless communication device, and a wireless communication method which attain increase in transmission capacity in spatial multiplexing communication.
Specifically, the present invention relates to a wireless communication system, a wireless communication device, and a wireless communication method which attain increase in transmission capacity in spatial multiplexing communication without broadening a range of frequencies, increasing a modulation multivalued number, and increasing circuit size. The present invention particularly relates to a wireless communication system, a wireless communication device, and a wireless communication method which attain increase in transmission capacity utilizing an algorithm of an MIMO transmission method.
2. Description of the Related Art
In recent years, wireless networks have been getting attention as systems free from wiring which has been used in wired communication methods in the related art. Examples of standards of wireless LANs (Local Area Networks) include an IEEE (The Institute of Electrical and Electronics Engineers) 802.11 standard and an IEEE 802.15 standard.
For example, an IEEE 802.11 a/g standard employs an OFDM (Orthogonal Frequency Division Multiplexing) modulation method which is one of multicarrier methods. In the OFDM modulation method, since data blocks are transmitted so as to be distributed to a plurality of carriers having predetermined frequencies orthogonal to one another, the carriers have narrow bands, high frequency efficiency is attained, and high frequency selective fading resistance is obtained.
Furthermore, although the IEEE 802.11 a/g standard supports modulation methods which realize a communication speed of 54 Mbps at a maximum, there is a demand for a next-generation wireless-LAN standard which realizes a higher-bit rate.
As a technique which realizes speeding-up of wireless communication, MIMO (Multi-Input Multi-Output) communication has been getting attention. MIMO communication is a communication method which employs a transmitting device and a receiving device each of which includes a plurality of antenna elements so as to realize spatially multiplexed streams. In the transmitting device, a plurality of transmission data blocks are subjected to multiplexing by performing space/time encoding, and the multiplexed transmission data blocks are distributed to a plurality of transmission antennas so as to be transmitted to channels. On the other hand, in the receiving device, signals received using a plurality of reception antennas via the channels are subjected to space/time decoding so as to obtain reception data blocks without crosstalk between streams.
According to the MIMO communication method, increased transmission capacity is obtained by increasing the number of antennas without broadening a range of frequencies. Accordingly, improved communication speed is attained. Furthermore, since space multiplexing is employed, high frequency efficiency is achieved. The MIMO method is a communication method utilizing channel characteristics, and therefore, the MIMO method is different from a simple transmission/reception adaptive array.
Here, in order to further increase the transmission capacity in the wireless communication system (that is, in order to improve the frequency efficiency), the following three methods may be employed.
(1) A range of frequencies is broadened.
(2) A modulation multivalued number is increased.
(3) The number of reception antennas is increased in order to increase the number of streams subjected to spatial multiplexing.
However, there arises the following technical problems in a case where each of the three methods is employed in the MIMO communication system in the related art.
(1) According to a frequency allocation method, frequency bands other than frequency bands in ISM (Industrial Scientific and Medical) band used in recent systems cannot be used. For example, a frequency band used in the wireless LAN systems in the related art is 20 MHz or 40 MHz.
(2) An EVM (Error Vector Magnitude) value required for an amplitude-modulation phase-modulation method of 256 QAM (Quadrature Amplitude Modulation) or more is not obtained due to imperfection of an RF circuit. Accordingly, since, in the wireless communication systems in the related art, QAM values up to 64 QAM are used, it is difficult to increase the modulation multivalued number.
(3) In a MIMO transmitting device, a size of a circuit used to perform spatial multiplexing or spatial demultiplexing linearly increases N-times or more in accordance with the number N of streams which are to be subjected to the spatial multiplexing. Furthermore, in an environment of transmission in which a transmission distance is small and an arrival angle is small, such as indoor spaces, the number of independent channels which is expected by the MIMO is four at most.
That is, in any of three methods (1) to (3), increase in transmission capacity may not be expected.
For example, a wireless transmission device capable of further improving frequency efficiency and a transmission rate while communication quality is maintained has been proposed (refer to Japanese Unexamined Patent Application Publication No. 2003-204317, for example). In the wireless transmission device, in order to control transmission frequencies of a plurality of antennas and the number of information items to be transmitted from the plurality of antennas in accordance with conditions of transmission channels, for example, transmission of different information items from a plurality of antennas using an identical frequency (spatial multiplexing), transmission of different information items from a plurality of antennas using different frequencies (frequency multiplexing), transmission of identical information items from a plurality of antennas using an identical frequency (spatial diversity), and transmission of identical information items from a plurality of antennas using different frequencies (frequency diversity) are appropriately switched to one another in accordance with the conditions of the transmission channels.
Furthermore, a receiving device which realizes frequency efficiency increased the same times as the number of multiplexing of transmission signals, a high speed, and increased transmission capacity by multiplexing a plurality of different information signal systems on an identical frequency using a MIMO channel which realizes broadband mobile communication and by cancelling interference using a receiving device has been proposed (refer to Japanese Unexamined Patent Application Publication No. 2005-252602, for example).
Moreover, a method for obtaining increased transmission capacity in an entire MIMO communication system which transmits a plurality of streams by spatial multiplexing transmission by assigning modulation methods optimum for individual streams has been proposed (refer to Japanese Unexamined Patent Application Publication No. 2005-323217, for example).
However, in the MIMO communication systems described above, the number of antennas should be increased or modulation degrees for individual streams should be increased in order to attain increased transmission capacity. However, there is a limit to increase transmission capacity due to capability of an RF circuit and a cost problem.